Organic peroxides play an important role as initiators in the preparation of polymers or as oxidizers in medical preparations and complex chemical syntheses.
Organic peroxides are thermally sensitive, highly reactive compounds known to decompose in a self accelerating exothermic reaction when not kept at a low enough temperature. Onset and progress of a respective self accelerating reaction, depend not only on the temperature, but also on the heat dissipation conditions at which a respective organic peroxide is kept. A SADT (Self Accelerating Decomposition Temperature) defining the lowest temperature at which the exothermic decomposition may start, thus does not represent an absolute value but reflects also the conditions under which the respective organic peroxide is kept. Smaller packages usually have a higher surface/volume ratio than bigger packages, and therefore better heat dissipation conditions which result in higher SADTs.
Due to their thermal instability, a synthesis of organic peroxides requires a very precise temperature control to avoid any serious incidents. Since the respective production or preparation processes use a bi-phase or multiphase reaction of immiscible phases, a thorough mixing of the reaction components is required to achieve a satisfactory reaction rate.
Organic peroxides may be produced in discontinuous or continuous processes. In discontinuous processes, the reactants are either all loaded into a reactor (batch reaction) or one reactant or catalyst is dosed to the other reactants provided in a reactor (semi batch reaction). The ratio between the reaction volume and the cooling surface available in such reactors is usually high, making a precise temperature control difficult and thus limiting the amount of organic peroxides to be produced safely within one lot.
For larger production volumes, continuous preparation processes are therefore preferred, where the supply of the starting materials and the extraction of the final product occurs on a continuous basis.
L. Fritzsche and A. Knorr describe in “Transformation of the 2nd step of a peroxyester synthesis from semi-batch to continuous mode”, Chemical Engineering and Processing 70 (2013) 217-221, a transformation of organic peroxide synthesis from semi-batch to continuous mode. The reaction is performed in a tubular continuous flow reactor exposed to ultrasound for improved mixing and increase of interface of phase boundary and thus better mass transfer between the two phases.
In “Continuous synthesis of a high energetic substance using small scale reactors”, Chemical Engineering Transactions, 32 (2013) 685-690, L. Fritzsche and A. Knorr describe a “split-and-recombine” (SAR) reactor where two meandering channels repeatedly split and recombine along the whole of their lengths. The SAR reactor is used for TBPEH (tert-Butyl peroxy-2-ethylhexanoate) synthesis with application of ultrasound.
Document WO 2008/006666 A1 e.g. describes a continuous process for the preparation of acyl peroxides using a reactor with two reaction zones. The first reaction zone is configured as a loop reactor where most of the bi-phase reaction mixture is circulated in a cooled loop, while part of it, or more precisely between 20% and 50% of the circulating volume, is sup-plied to the second reaction zone and replaced by a corresponding amount of newly fed starting materials. The second reaction zone is formed by a stirred cell reactor where two or more reaction cells are connected in series with the content of each of the reaction cells being mixed by at least one stirrer. The reaction cells are connected to one another such that there is virtually no backmixing of the reaction mixture from a downstream reaction cell into an upstream reaction cell. Though enabling a continuous preparation of organic peroxides, the processing in the second reaction zone represents a sequence of CSTRs (continuous stirred tank reactors) rather than a continuous flow-through reactor, since the second reaction zone is organized in a cascade of cells with each cell processing a certain part of the total reaction volume for a specified period. Due to the cell processing, the ratio between the cooling surface and the volume of the reaction mixture is in the second reaction zone still comparatively poor, thus limiting the throughput of the reactor or requiring a large number of reaction cells. Different to a continuous flow reaction, the stirring results in a remixing of portions where the conversion is in an advanced state with portions where the conversion is still poor. Due to this, CSTRs require numerous cells to get a good final conversion. As a further consequence of the remixing, the mechanical stirrers provide no finely dispersed mixing of the phases resulting in a comparatively low conversion and/or long residence time (time required for the reaction mixture to pass through the reactor).
Flow-through reactors such as, for instance, tube reactors, plate reactors or the like enable a continuous preparation of organic peroxides in a continuous flow. Flow-through reactors comprise at least one reactor channel for the reaction mixture to pass through, whereby when more than one reactor channel is used, the channels can be connected in parallel and/or in series. Due to the reaction mixture flowing continuously through the reactor channels, a local concentration of the reaction components is basically a function of the distance traversed by the reaction mixture along the length of the reactor channel(s) to the respective position, and can be described by way of the plug flow reactor model. In other words, the concentration of the reaction mixture components is assumed to change only along its flow direction while having no gradients transverse to the flow direction.
Phase mixing in flow-through reactors is usually accomplished by creating turbulences, i.e. irregular local flows with directions different to the main flow direction. Turbulences can either be created by means of high flow rates (usually characterized by a Reynolds number above 3,000 or so) or by introducing redirection means into the flow path like baffles (see e.g. WO 1999/55457 A1), or by channel wall irregularities (e.g. helical protrusions or indentations as disclosed in WO 2006/092360 A1), or by changes in the reaction channel or flow path direction (as for instance described in WO 2012/095176 A1) or by splitting and recombining the flow (e.g. herring bone structures as described in WO 2014/044624 A1). Turbulences not only provide a thorough mixing of the immiscible phases present in the synthesis of organic peroxides, but usually also result in smaller maximal droplet sizes than are possible with mechanical agitation means like stirrers or the like. Smaller maximal droplet sizes in turn provide larger reaction surfaces causing higher reaction rates and thus shorter reaction times. Since a mixing based on turbulences requires no moving parts, respective reactors are also referred to as static mixers.
Process conditions for synthesizing organic peroxides can be improved by using so-called mini-reactors. Mini-reactors are characterized by having flow channel dimensions (transverse to the main flow direction) in the millimeter (milli-reactors) or even in the micrometer (micro-reactors) range. A use of mini-reactors reduces the local reaction volume significantly, while increasing at the same time the ratio between the reactor channel surface available for cooling and the reactor channel volume. This enables an improved control of local reaction temperatures that, together with the smaller local volumes, improves the safety of the preparation process.
One example for a continuous flow-through mini reactor is a plate exchanger as e.g. described in document WO 2007/125091 A1. The plate exchanger comprises three plates arranged to form reactor channels and heat exchange channels between them. The reactor can be used for synthesizing organic peroxides for which two reactor channels are connected in series. Two reactants are fed to a first of the two reactor channels to form an intermediary product which is then fed to the other of the two reactor channels together with a third reactant to form the final product. A heat transfer fluid runs through the heat exchange channels for dissipating the reaction heat.
Another example for a continuous flow-through mini reactor is disclosed in WO 2014/044624 A1. The reactor comprises at least two comb-like structures with angled teeth. One of the two structures is disposed on top of the other such that the teeth of the two structures cross over. The thus combined structures are placed into a housing covering its top and bottom faces to form crossing pathways along which a fluid is forced to change its flow direction repeatedly. For enabling a preparation of organic peroxides, the housing is placed in a tube passed through by a cooling liquid.
The flow-through mini reactor disclosed in WO 2012/095176 A1 provides a reaction channel, which pathway directions change repeatedly. The reaction channel is formed in a plate cov-ered by a further plate. To intensify the turbulences and thus improve the mixing of the processed fluid, an oscillatory flow is superimposed on the fluids steady flow.
The oscillatory flow is limited to a region between the inlet for the starting materials and the outlet for the final product and results in recurring high flow rates inside the reactor. The term “oscillatory flow” signifies a variation of the flow rate with time, whereby the average flow rate of an oscillatory flow equals zero. When superimposing an oscillatory flow on a steady flow, the average flow rate is thus given by the rate of the steady flow. Due to the temporary higher flow rates, however, stronger turbulences are created that result in a more efficient mixing of the reaction mixture components. The residence time of the reaction mixture is not affected by the oscillatory flow, since the average flow rate still equals the steady flow rate.
A superimposition of an oscillatory flow on a steady flow has already been described in patent specification U.S. Pat. No. 4,271,007 as a proper means for preventing a deposition of solids on the walls of a tubular reactor used for high-temperature hydrocarbon cracking. The oscillation frequency used was 115 Hz. WO 2012/095176 A1 describes a use of an oscillatory flow in a mini-reactor which reactor channel is configured with repeated pathway changes. The oscillatory flow is superimposed on a steady flow to effectively mix a suspension being processed such that a deposition of solid material in the reactor is prevented and no sedimentation, fouling or clogging of the reactor has to be worried about.
A preparation of organic peroxides in continuous flow reactors is at present effected under steady flow conditions where the local flow rates do not change with time. To achieve a required mixing of the components in the reaction mixture, the flow rate of the reaction mixture has to be high enough to cause turbulent flow conditions. In this context it is noted that although turbulences introduce chaotic flow conditions, the flow rate through a section of the reactor (a length of the flow channel) does not generally change with time, and the term steady flow conditions is in this document therefore also used for turbulent flows, where the pattern of the fluid's movement along the length of the reactor's pathway does not change with time. The term “steady flow” as used in this specification refers not only to a (possibly slowly and/or slightly changing) constant flow but also to more or less periodic intermittent flow characteristics like pulsating flows, which intermission periods are much shorter than the reaction time, e.g. by a factor of ten or more.
Since a synthesis of organic peroxides has to be carried out at relatively low temperatures, the reaction times required are comparatively long. In order to complete a respective synthesis to the desired extent in a flow-through reactor operated under steady flow conditions, the reaction mixture has to reside in the reactor for the whole of the reaction time required. The length of time that elapses between an introduction of starting materials and the output of a final product synthesized from these starting materials is called residence time and corresponds to the above reaction time. Together with the flow rate of the reaction mixture, this time period defines the length of the reaction pathway required. The higher the necessary flow rate and the longer the residence time, the longer the flow channel defining the pathway.
A synthesis of organic peroxides also requires an efficient temperature control of the reaction medium. The lateral dimensions of the flow channel, i.e. its dimensions transverse to the flow direction defined by it, have therefore to be, at least in one direction, small enough to guar-antee an effective heat transfer. Due to this cross sectional limitations, a long flow channel accordingly implies a high flow resistance. High flow resistances in turn result in considerable pressure drops that are difficult to handle and can make an implementation of a respective reactor a technical challenge. In addition, long reaction flow channels also imply large reaction volumes which in the case of peroxides give rise to serious risks.
There is therefore a desire for a process and an apparatus enabling a safe large-scale synthesis of organic peroxides under continuous flow-through conditions.